Why on earth would you want to run less boost, I mean doesn’t more boost equal more power?
Well…yes and no. It is actually possible to make more power with less boost, but changes are required.
By now, everyone should understand the relationship between boost and power. It should come as no surprise that it is almost always possible to make more power by raising the boost applied to a motor. The question is, how then, can the opposite also be true?
The answer requires understanding that there is more to power production than boost, and that pressure alone should never be considered a true indication of power. Though we often label it “positive pressure,” boost is more accurately an indication of back pressure built up in the intake track. Yes folks, that wonderful boost gauge is actually reading how much restriction your engine represents to the airflow provided by (in this case) the supercharger.
This test involved a very simple principle: Let’s rid the system of said restrictions, then watch the power climb. As a side benefit, the boost pressure will also drop.
Expressed as a simple equation to determine horsepower, we combine boost plus the power output of the engine itself. To increase power, you can alter either of the two components in the equation. Increased power can be accomplished by increasing boost pressure from the supercharger OR increasing the power output of the normally aspirated motor you are applying the boost to. You can of course opt to do both, but we kept things simple by altering only the engine side of the equation for this test.
What could be better than running a Vortech supercharger on one LS application? Easy, running the blower on two LS motors! (Image/Richard Holdener)
Comparison Testing
The boost provided by a supercharger is a function of the impeller speed relative to the engine speed. Coupled to this equation is the power output of the normally aspirated motor.
To illustrate what happens to power and boost when we alter the engine side of the equation, we ran a Vortech Ti-trim supercharger on two different test motors. The Vortech was run at the same impeller speed (using the same blower and crank pulleys) with the same intercooler and at the same air/fuel and timing values. The only variable in the equation was the displacement and power output of the two test motors.
The Test Motors
Test motor number one was a mild 4.8L equipped with a stock block, stock crank and rods, but fortified somewhat with JE forged pistons. To this we added a set of CNC-ported, Trick Flow GenX 205 cylinder heads and mild Stage 1 Truck Cam from Brian Tooley Racing.
Test motor number two was a larger 6.0L equipped with CP/Carrillo forged pistons (and Total Seal piston rings), TEA Stage 2 243 heads and a healthy Comp cam. Both motors were run with the same early truck intake, 75 pound injectors and FAST XFI/XIM engine management system.
The Dyno Test Results
Before adding boost, we ran each motor in normally aspirated trim to help illustrate both the gains offered by the impressive Vortech supercharger and to see if the difference in power normally aspirated remained constant once we added boost.
First up on the dyno was the mild 4.8L. Sporting the mild BTR Truck cam and TFS heads, the 4.8L produced 398 hp and 353 lb.-ft. of torque. Adding the Vortech supercharger to the mix with the 3.8/7.5 pulley combo and air-to-water intercooler resulted in peak numbers of 746 hp and 592 lb.-ft. of torque. It should be noted that we ran the 4.8L slightly higher than the 6.0L (6,700 rpm), so we will make our comparison at the peak engine speed of 6,500 rpm. Boost supplied by the Vortech started out at 3.1 psi at 3,100 rpm then rose to a peak of 14.6 psi at 6,500 rpm (15.2 psi at 6,700 rpm).
Using the Vortech supercharger, we nearly doubled the power output of the little 4.8L, but now it was time to dyno test the larger 6.0L.
Off came the 4.8L and on went the more powerful 6.0L. Thanks to increased displacement, compression and cam timing, the 6.0L offered considerably more power than the smaller 4.8L in normally aspirated trim. Run on the dyno with the same Fast XFI/XIM management system, the 6.0L produced 549 hp and 481 lb.-ft. of torque. After adding the Vortech blower to the mix, the peak numbers skyrocketed to 896 hp and 724 lb.-ft. of torque.
The power certainly increased, but what happened to the boost?
The boost curve started out at 2.9 psi at 3,100 rpm and rose to a peak of 12.8 psi at 6,500 rpm. Run on the larger 6.0L sporting an extra 150 hp (over the NA 4.8L), the peak boost dropped by 1.8 psi. The change in boost between the two motors increased with engine speed, so the difference would be even greater at 7,000 rpm. The benefit of adding more motor instead of adding more boost is that a reduction in boost will decrease the charge temperature and chance of detonation, while an increase in boost will do the opposite.
Of course, you can always do both and crank up the boost on the bigger motor for maximum effect—just know that it is possible to get more power with less boost.
Before illustrating the effect of increased normally aspirated power on boost, we had to test each of the two combinations in normally aspirated trim. We ran both the mild 4.8L and slightly wilder 6.0L on the engine dyno. Run in normally aspirated trim with TFS heads and mild BTR Stage 1 Truck cam, the 4.8L produced 398 hp at 6,300 rpm and 353 lb.-ft. of torque at 5,600 rpm. By contrast, the larger and more powerful 6.0L produced 549 hp at 6,900 rpm and 481 lb.-ft. at 5,000 rpm. Even before adding boost, the 6.0L offered an additional 150 hp over the smaller 4.8L. The question now was what would happen when we added the Vortech supercharger to the mix? (Dyno Chart/Richard Holdener)Both the 4.8L and 6.0L were equipped with the same Vortech supercharger kit, right down to the pulley combination and air-to-water intercooler. After running on the smaller 4.8L, every component of the Vortech supercharger kit was pulled and installed onto the awaiting 6.0L, including the 3.80 inch blower pulley and 7.5-inch ATI damper/crank pulley. The test was designed to illustrate the change in power and boost pressure between the two motors using the same impeller speed. Run on the 4.8L, the Vortech produced peak numbers of 746 hp at 6,700 rpm and 592 lb.-ft. of torque at 6,300 rpm. Running the same combination on the larger 6.0L resulted in 896 hp at 6,500 rpm and 724 lb.-ft. of torque at 6,400 rpm. Measurements taken at the same engine speed indicate the same 150 hp difference between the two combinations. (Dyno Chart/Richard Holdener)The net result of increasing the displacement and power output of the test motor was a decrease in boost pressure. Once again measured at the same engine speed of 6,500 rpm, the increased power output of the larger 6.0L dropped boost pressure by 1.8 psi. Note that the change in boost increased with engine speed, starting at just 0.2 psi difference at 3,100 rpm then increasing to 1.8 psi at 6,500 rpm. This increasing trend would continue had we elected to increase the engine speed during testing. (Dyno Chart/Richard Holdener)Test mule number one was this mild 4.8L equipped with a stock block, crank, and Gen. 4 rods, but augments with JE forged (small dome) pistons. (Image/Richard Holdener)The stock 4.8L cam was replaced with a mild Stage 1 truck cam from Brian Tooley Racing. The BTR Stage 1 Truck cam offered 0.553 lift, a 206/212 degree duration split and 112 degree LSA. (Image/Richard Holdener)Topping the mild-cammed 4.8L was a set of Trick Flow GenX 205 heads. The CNC-ported heads are likely the best thing going for the small-bore (3.78 inch) 4.8L and 5.3L applications. (Image/Richard Holdener)To ensure adequate fuel delivery, the factory injectors were replaced by 75 pounders from Comp Cams. (Image/Richard Holdener)Both the 4.8L and 6.0L were tuned using the Fast XFI/XIM management system. (Image/Richard Holdener)Before adding the supercharger, we ran the 4.8L motor in naturally aspirated trim. The mild 4.8L combo produced 398 hp and 353 lb.-ft. of torque. (Image/Richard Holdener)Each of our two test motors received a Vortech kit that included this Ti-trim supercharger. (Image/Richard Holdener)The supercharger kit was configured with a ten-rib, 3.80 inch blower pulley. (Image/Richard Holdener)To keep charge temps down, we configured the supercharged LS with this air-to-water intercooler. The cooler was plumbed with 88 degree dyno water. (Image/Richard Holdener)Run with the Vortech, the supercharged 4.8L produced 746 hp and 592 lb.-ft. of torque at a peak boost level of 15.2 psi at 6,700 rpm (14.6 psi at 6,500 rpm). (Image/Richard Holdener)Next up we installed a larger, more powerful 6.0L. The 6.0L featured forged CP/Carrillo Bullet series rods and pistons. (Image/Richard Holdener)The 6.0L featured wilder cam timing than the 4.8L. Right from the catalog, the hydraulic roller from Comp Cams offered a 0.617/0.624 lift split, a 231/247 degree duration split, and 113 degree LSA. (Image/Richard Holdener)Like the 4.8L, the 6.0L was treated to exceptional head flow in the form of a set of Stage 2 243 heads from Total Engine Airflow. (Image/Richard Holdener)Both the 4.8L and 6.0L shared the same induction system consisting of a factory (early) truck intake was run with the early truck intake and Accufab throttle body. (Image/Richard Holdener)The extra displacement of the 6.0L combined with substantial head flow and wilder cam timing to produce an extra 150 hp over the smaller 4.8L. In normally aspirated trim, the 6.0L produced 549 hp and 481 lb.-ft. of torque. (Image/Richard Holdener)In addition to the 3.80 inch blower pulley, each combination was run with this 7.5 inch ATI Super Damper. Each was pinned to the factory crank to eliminate damper slippage under boost. (Image/Richard Holdener)Run with the same Vortech kit and intercooler, the supercharger 6.0L produced 896 hp and 724 lb.-ft. of torque. In supercharged form, the 6.0L offered an additional 150 hp and dropped the boost pressure by 1.8 psi, meaning we did indeed get More Power with Less Boost! (Image/Richard Holdener)
Richard Holdener is a technical editor with over 25 years of hands-on experience in the automotive industry. He's authored several books on performance engine building and written numerous articles for publications like Hot Rod, Car Craft, Super Chevy, Power & Performance, GM High Tech, and many others.
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